Could global warming increase wind shear over the Atlantic, potentially leading to a decrease in the frequency of Atlantic hurricanes? There is a growing consensus among hurricane scientists that this is indeed quite possible. Two recent studies, by Zhao et al. (2009), "Simulations of Global Hurricane Climatology, Interannual Variability, and Response to Global Warming Using a 50-km Resolution GCM", and by Knutson et al. (2008), "Simulated reduction in Atlantic hurricane frequency under twenty-first-century warming conditions", found that global warming might increase wind shear over the Atlantic by the end of the century, resulting in a decrease in the number of Atlantic hurricanes. For example, the second study took 18 relatively coarse (>60 km grid size) models used to formulate the 2007 IPCC climate report, and "downscaled" them using a higher-resolution (18 km grid size) model called ZETAC that was able to successfully simulate the frequencies of hurricanes over the past 50 years. When the 18 km ZETAC model was driven using the climate conditions we expect in 2100, as output by the 18 IPCC models, the authors found that a reduction of Atlantic tropical storms by 27% and hurricanes by 18% by the end of the century resulted. An important reason that their model predicted a decrease in the frequency of Atlantic hurricanes was due to a predicted increase in wind shear. As I explain in my wind shear tutorial, a large change of wind speed with height over a hurricane creates a shearing force that tends to tear the storm apart. The amount of wind shear is critical in determining whether a hurricane can form or survive.

Figure 1. Top: predicted change by 2100 in wind shear (in meters per second per degree C of warming--multiply by two to get mph) as predicted by summing the predictions of 18 climate models. Bottom: The number of models that predict the effect shown in the top image. The dots show the locations where tropical storms formed between 1981-2005. The box indicates a region of frequent hurricane formation where wind shear is not predicted to change much. Image credit: Geophysical Research Letters, "Increased Tropical Atlantic Wind Shear in Model Projections of Global Warming", by Vecchi and Soden, 2007.

Since the Knutson et al. study using the 18 km resolution ZETAC model was not detailed enough to look at what might happen to major Category 3 and stronger hurricanes, a new study using a higher resolution model was needed. This was done by a team of modelers led by Dr. Morris Bender of NOAA's GFDL laboratory, who published their results in Science in February. The authors used the GFDL hurricane model--the model that has been our best-performing operation hurricane track forecasting model over the past five years--to perform their study. The GFDL hurricane model runs at a resolution of 9 km, which is detailed enough to make accurate simulations of major hurricanes. The researchers did a double downscaling study, where they first took the forecast atmospheric and oceanic conditions at generated by the coarse (>60 km grid) IPCC models, used these data to initialize the finer resolution 18 km ZETAC model, then used the output from the ZETAC model to initialize the high-resolution GFDL hurricane model. The final results of this "double downscaling" study suggest that although the total number of hurricanes is expected to decrease by the end of the century, we should expect an increase of 81% in the number of Category 4 and 5 storms in the Atlantic. This trend should not be clearly detectable until about 60 years from now, given a scenario in which CO2 doubles by 2100. The authors say that their model predicts that there should already have been a 20% increase in the number of Category 4 and 5 storms since the 1940s, given the approximate 0.5°C warming of the tropical Atlantic during that period. This trend is too small to be detectable, given the high natural variability and the difficulty we've had accurately measuring the exact strength of intense hurricanes before the 1980s.The region of the Atlantic expected to see the greatest increase in Category 4 and 5 storms by the year 2100 is over the Bahama Islands (Figure 2), since wind shear is not expected to increase in this region, and sea surface temperatures and atmospheric instability are expected to increase there.

The net effect of a decrease in total number of hurricanes but an increase in the strongest hurricanes should cause an increase in U.S. hurricane damages of about 30% by the end of the century, the authors compute, assuming that hurricane damages behave as they did during the past century. Over the past century, Category 4 and 5 hurricanes made up only 6% of all U.S. landfalls, but accounted for 48% of all U.S. damage (if normalized to account for increases in U.S. population and wealth, Pielke et al., 2008.)

Figure 2. Expected change in Atlantic Category 4 and 5 hurricanes per decade expected by the year 2100, according to the Science paper by Bender et al. (2010).

CommentaryThese results seem reasonable, since the models in question have been successfully been able to simulate the behavior of hurricanes over the past 50 years. However, the uncertainties are high and lot more research needs to be done before we can be confident of the results. Not all of the IPCC models predict an increase in wind shear over the tropical Atlantic by 2100, so the increase in Category 4 and 5 hurricanes could be much greater. Also, the GFDL model was observed to under-predict the strength of intense hurricanes in the current climate, so it may not be creating enough Category 4 and 5 hurricanes in the future climate of 2100. On the other hand, IPCC models such as the UKMO-HadCM3 predict a very large increase in wind shear, leading to a drastic reduction in all hurricanes in the Atlantic by 2100, including Category 4 and 5 storms. So Category 4 and 5 hurricane frequency could easily be much greater or much less than the 81% increase by 2100 found by Bender et al.

The estimates of a 30% increase in hurricane damages by 2100 may be considerably too low, since this estimate assumes that sea level rise will continue at the same pace as was observed in the 20th century. Sea level rise has accelerated since the 1990s, and it is likely that this century we will see much more than than the 7 inches of global sea level rise that was observed last century. Higher sea level rise rates will sharply increase the damages due to storm surge, which account for a large amount of the damage from intense Category 4 and 5 hurricanes.

Keep in mind that while a 30% in hurricane damage by the end of the century is significant, this will not be the main reason hurricane damages will increase this century. Hurricane damages are currently doubling every ten years, according to Pielke et al., 2008. This is primarily due to the increasing population along the coast and increased wealth of the population. The authors theorize that the Great Miami Hurricane of 1926, a Category 4 monster that made a direct hit on Miami Beach, would have caused about $150 billion in damage had it hit in 2005. By 2015, the authors expect the same hurricane would do $300 billion in damage. This number would increase to $600 billion by 2025 (though I think it is likely that the recent recession may delay this damage total a few years into the future.) It is essential that we limit coastal development in vulnerable coastal areas, particularly along barrier islands, to reduce some of the astronomical price tags hurricanes are going to be causing. Adoption and enforcement of strict building standards is also a must.

The authors of the GFDL hurricane model study have put together a nice web page with links to the paper and some detailed non-technical explanations of the paper.

Quoting hydrus: I saw a lot of upwelling from Hurricane Bill and a large area of ocean had cooled off substantially. I remember thinking that if a storm were to move over the cooler water the storm would have weakened.

Oh indeed. It happens a lot. In one of our analog years, 1998, Major Hurricane Bonnie moved slowly north of the Bahamas and caused major upwelling. Just a few days later, Hurricane Danielle transversed over the same area north of the Bahamas, hit the cold water wake, and was limited to Cat 1 intensity until she deviated from Bonnie's course.

That can be true in the SW Atlantic, but it's very hard to do that to the Caribbean in the height of the hurricane season. Look at how deep the 26C isotherm is already at this time of year:

It's even more impressive in August and September. Hurricanes end up upwelling warm water instead of cold water like they normally would anywhere else. The gulf can see upwelling from multiple strong storms, but not over the Loop Current. The point is it would take a huge amount of energy to drain enough heat from the Caribbean water to not be able to support a major hurricane.

I saw a lot of upwelling from Hurricane Bill and a large area of ocean had cooled off substantially. I remember thinking that if a storm were to move over the cooler water the storm would have weakened.

That's why the cold water in the north Atlantic between 30N and 45N scares me. The European is seeing it, and focusing activity to the south accordingly, aimed at the entire Caribbean and US coastline in that image you posted. Not a pretty picture to be looking at.

I have a question Levi. Can that colder water between 30 & 45 degrees cause any high pressure ridging?

Quoting hydrus: If the storms keep tracking in the same general path, the water will be cooled and some of the storms should be weak. But the first 5 or so could be potent because of those above average sea temps.

That can be true in the SW Atlantic, but it's very hard to do that to the Caribbean in the height of the hurricane season. Look at how deep the 26C isotherm is already at this time of year:

It's even more impressive in August and September. Hurricanes end up upwelling warm water in the Caribbean instead of cold water like they normally would anywhere else. The gulf can see upwelling from multiple strong storms, but not over the Loop Current. The point is it would take a huge amount of energy to drain enough heat from the Caribbean water to not be able to support a major hurricane.

That rule sure didn't apply in 2005. It highly depends on what is making a lot of storms form. Lots of times the high numbers result from random tropical storms forming out near the Azores or in the southern Bay of Campeche, not in areas where they can become very strong, and so they contribute to the numbers but not so much to the ACE. Even 2005 had a whole lot of storms form way out in the middle of the north Atlantic due to all the warm water, away from the main congregation of stronger storms. However, a season like this with cold water in the north Atlantic has the potential to focus most of the formations in the deep tropical breeding grounds, which I think could make the ACE quite high relative to the storm numbers. We shall see.

If the storms keep tracking in the same general path, the water will be cooled and some of the storms should be weak. But the first 5 or so could be potent because of those above average sea temps.

Yeah~ for 6 weeks or more I've held my tongue on condemning them to the brunt & still don't want to. Your right, a lot can change in 2 months. Just don't like these models jumping on board.

I do think the Japanese is way too fast in dropping this off, but the Euro may still have it right based on the SOI skyrocketing right now. The drop is delayed from all the model forecasts right now because of that one Kelvin wave burst in February, but all that's over now, and it should start coming down steadily on average over the next couple months.

Another thing I'm looking at...may or may not have merit...but if this pans out, where are systems most likely to develop? and if so, where is the most likely tracks to be?

That's why the cold water in the north Atlantic between 30N and 45N scares me. The European is seeing it, and focusing activity to the south accordingly, aimed at the entire Caribbean and US coastline in that image you posted. Not a pretty picture to be looking at.

That has been in my mind as well. How fast the ENSO falls off is one of the things that is still up in the air and may determine where the heavy congregation of tracks may be this year. Based on the SST anomalies I'm afraid the Caribbean may have a bad year in any case, but there is still a lot to be figured out in the next 2 months.

Yeah~ for 6 weeks or more I've held my tongue on condemning them to the brunt & still don't want to. Your right, a lot can change in 2 months. Just don't like these models jumping on board.

Quoting NttyGrtty: So normal may be abnormal? lol! Personally, I don't think ACE is being giving enough weight. If we have more storms, they won't be as strong, or if we have stronger storms, there will be less of them. We shall see what we shall see...

That rule sure didn't apply in 2005. It highly depends on what is making a lot of storms form. Lots of times the high numbers result from random tropical storms forming out near the Azores or in the southern Bay of Campeche, not in areas where they can become very strong, and so they contribute to the numbers but not so much to the ACE. Even 2005 had a whole lot of storms form way out in the middle of the north Atlantic due to all the warm water, away from the main congregation of stronger storms. However, a season like this with cold water in the north Atlantic has the potential to focus most of the formations in the deep tropical breeding grounds, which I think could make the ACE quite high relative to the storm numbers. We shall see.

Indeed....hard to think of that as "average" but that's how it is during a warm AMO. All the numbers will be elevated during this period. A "normal" season now of 10-11 storms would actually be below-average if you take it in context.

So normal may be abnormal? lol! Personally, I don't think ACE is being giving enough weight. If we have more storms, they won't be as strong, or if we have stronger storms, there will be less of them. We shall see what we shall see...

Quoting Skyepony:Levi~ My thoughts have been if ENSO pulls a hard switch to La Niña, like the dynamic models are bent on, the Caribbean is going to have a really hard hitting season.

That has been in my mind as well. How fast the ENSO falls off is one of the things that is still up in the air and may determine where the heavy congregation of tracks may be this year. Based on the SST anomalies I'm afraid the Caribbean may have a bad year in any case, but there is still a lot to be figured out in the next 2 months.

Quoting NttyGrtty:1061. Levi, 10 year mathmatical average is 16/9/4. Proves nothing other than Excel actually runs the formulas typed into it but there you have it...

Indeed....hard to think of that as "average" but that's how it is during a warm AMO. All the numbers will be elevated during this period. A "normal" season now of 10-11 storms would actually be below-average if you take it in context.

Gujarat Global News Network, AhmedabadWith mercury touching 43 degrees in most of the cities, the state is witnessing record heat wave conditions. '; writethis(jsval);//--> It is almost after a decade that temperatures have rose to such an extent in the beginning of April. The met office has warned of heat wave conditions mostly in parts of North Gujarat and Kutch during the next 48 hours.

There has been significant rise in minimum temperatures also. Till Sunday the minimum temperature was around 20 degrees which has now shot upto 26 degrees. Idar in North Gujarat recorded the highest temperature of 44.6 degrees on Wednesday. It had minimum temperature of 31.6 degrees.

The new UKMET Seasonal forecast is out for March. This forecast is overall more scary than their February forecast in terms of how things set up in the beginning of the Atlantic hurricane season.

I believe they don't want their maps posted anywhere else, but you can go to their site and register to view the maps.

What I can tell you is that the general trend is the same on the model, forecasting lower-than-normal surface pressures in the Caribbean and Gulf of Mexico in June-July-August, but stronger negative anomalies than last month's forecast. The rainfall forecast is downright scary for the Caribbean, with a wide swath of very high precipitation anomalies aimed out of the MDR right through the Caribbean and into Central America.

The 500mb forecast continues to show a generally negative NAO look, with blocking over SE Canada. This is not a good pattern for the United States. I also took great interest to the fact that their temperature forecast is far warmer for the eastern CONUS than their February forecast. They do keep cooler-than-normal temperatures over the GOM and Florida during the JJA period, but they seem to be catching on more to the warm summer for most of the CONUS this year.

Another thing I noticed is that the UKMET seems to be leaning towards a very southern-focused season affecting the Caribbean more than anywhere else, based on the MSLP and precipitation anomaly forecasts. Their forecasts only go out to the JJA period, which is just the beginning of the season, so we may see that change, but it is an interesting trend to follow on the model.

Quoting hydrus:That chart showing all the tracks of the 2005 Hurricane Season is absolutely hideous. If something similar to that happens again, there will be some serious problems. On a positive note, the chances of having a season that devastating are low. jmo

Actually its quite possible all it takes is for 4 to 8 weeks of that perfect pattern and the U.S. coast is in deep trouble. Just like 2004 and 2005.